Rain gauges of the general type herein disclosed have been known for more than 30 years and have been illustrated and described in my U.S. Pat. No. 2,735,298 issued Feb. 12, 1956 and 2,908,165 issued Oct. 13, 1959 (on application filed Dec. 10, 1957). These patents both disclose the use of a syphon tube for periodically emptying the rain metering receptacle. In U.S. Pat. No. 2,735,298 upward movement of a float in the rain collecting receptacle is translated to a stylus which graphically records the level of collected rain on the recording sheet of a time controlled rotating drum; whereas U.S. Pat. No. 2,908,165 translates upward movement of the float to electrical impulses through an effective but somewhat intricate and cumbersome mechanism.
While the devices of these prior patents perform effectively in the macro sense they proved to be somewhat unreliable in the micro sense in that incremental upward movements of the float in the rain metering receptacle did not always provide a true indication of the quantity of rain being collected. This interfered with the accurate recording of the rate of rainfall within time intervals of special interest.
This problem in the micro responsiveness of the earlier instruments is brought about by irregularities in the functioning of the syphon tube in the periodic automatic emptying of the rain metering receptacle. It sometimes happens that at the end of a discharge cycle a slug of water will remain in the lower end of the discharge leg of the syphon tube. When this happens an entrapped volume of air in the syphon tube can interfere with flow of water into the intake end of the syphon tube with the result that for a time the rate of rise of water in the collecting receptacle may be a bit more rapid than an accurate measure of collected rainfall; and as pressure is built up in the air pocket within the syphon tube, a point will be reached when this pressure forces some or all of the water slug in the discharge leg to be ejected. When this happens the sudden rise of liquid in the intake leg of the syphon tube causes a momentary drop or a slowing of the rate of rise, of the liquid level in the metering receptacle. Depending upon the size of the water slug which had remained in the discharge leg at the end of a previous discharge cycle it could require one, two, or even three such build-up of pressure on the entrapped air pocket in the syphon tube to fully clear the water slug from the discharge leg; and with each such pressure build-up the rate of rise of collected rain in the metering receptacle is first increased and then momentarily decreased with respect to the true rate of rain addition to the metering receptacle.
It will be apparent that these unpredictable variations in rate of rise of collected rain in the receptacle present significant problems for those interested in accurate micro recording of the rate at which rain is falling. For this reason the device disclosed in my above mentioned patents have not attained broad acceptance in the market place.